Temperature corrected lab-on-a-chip-platform with integrated epoxy polymer Bragg gratings

We report on the fabrication technology and the application capability of an epoxy polymer based lab-on-a-chip (LOC) concept equipped with integrated Bragg grating sensors allowing for an on-chip temperature referenced refractive index (RI) measurement of fluids. Microfluidic channels as well as optical waveguide structures are fabricated in EpoCore / EpoClad epoxy-based photoresists on a Topas 6017 cyclo olefin copolymer (COC) substrate using multi-mask UV photolithography. Integrated optical sensor elements in terms of Bragg gratings are directly laser-written into the fabricated strip waveguides using the robust phase mask technique with optimized excimer laser parameters. While the thus produced sensors have turned out to suit well for evanescent refractive index sensing in humidity-saturated environments like aqueous solutions, however, the additional deployment as an independent on-chip fluid temperature sensor is widely restricted due to the considerably temperature-dependent moisture uptake and swelling mechanisms of the epoxy materials, which are comparable to the well-known issues in polymethylmethacrylate. These severe mechanisms prevent the physical quantities temperature and relative humidity from being measured independently. Hence, the presented chip-design demands an extra sensor element solely responding to temperature variations. In this contribution, we therefore propose and evaluate three differing approaches to realize an enhanced optofluidic lab-on-a-chip-platform with integrated temperature sensor for a corrected RI measurement using multiple Bragg grating sensors. In the first approach, an EpoCore waveguide Bragg grating sensor is symmetrically sandwiched in between two humidity insensitive Topas 6017 sheets by solvent bonding. In the second method, an amount of COC is fully dissolved in cyclohexane solvent and the liquid copolymer is drop coated onto the sensor area in terms of a thin layer aiming for humidity desensitization. The third approach uses the COC substrate directly itself as temperature sensor material. Thereby, besides the epoxy waveguide grating, a separated further waveguide Bragg grating is laser-written into the Topas substrate material being inherently insensitive to humidity influences.